Process for xylenes isomerization

ABSTRACT

A process for the isomerization of a para-xylene depleted, meta-xylene rich stream under at least partially liquid phase conditions using ZSM-23 with an external surface area of at least 75 m 2 /g (indicating a small crystallite size), and a SiO 2 /Al 2 O 3  ratio between 15 and 75 that produces a higher than equilibrium amount of para-xylene, i.e., more than about 24 wt % of para-xylene, based on the total amount of xylenes.

PRIORITY

This invention claims priority to and the benefit of U.S. Ser. No.62/267,428, filed on Dec. 15, 2015, which is incorporated by referenceherein.

FIELD OF THE INVENTION

This invention relates to an improved xylenes isomerization process.

BACKGROUND OF THE INVENTION

Para-xylene (PX) is a valuable chemical feedstock, which may be derivedfrom mixtures of C₈ aromatics separated from such raw materials aspetroleum naphthas, particularly reformates. The C₈ aromatic fractionsfrom these sources vary quite widely in composition but, in the case ofa reformate stream, will usually comprise 10 to 32 wt % ethylbenzene(EB) with the balance, xylenes, being divided between approximately 50wt % of meta-xylene (MX) and 25 wt % each of para-xylene andortho-xylene (OX). Of these isomers, para-xylene is by far the mostimportant for commercial applications.

Individual isomer products may be separated from the naturally occurringC₈ aromatic mixtures by appropriate physical methods. Ethylbenzene maybe separated by fractional distillation, although this is a costlyoperation. Ortho-xylene may be separated by fractional distillation, andis so produced commercially. Para-xylene may be separated from the mixedisomers by fractional crystallization, selective adsorption or simulatedmoving bed chromatography (e.g., the Parex™ or Eluxyl® process), ormembrane separation.

As commercial use of para-xylene has increased, combining physicalseparation with chemical isomerization of the other xylene isomers toincrease the yield of the desired para-isomer has become increasinglyimportant. Prior art commercial processes separate para-xylene from theother xylene isomers and isomerize the para-depleted stream over ZSM-5,producing an equilibrium mixture of xylenes, which contains about 23 wt% or less of para-xylene, based on the total amount of xylenes in theisomerized stream. The isomerized stream is then recycled to thepara-xylene separation step, forming what is commonly known as thexylenes loop. Because of the relatively low amount of para-xyleneproduced in the xylenes loop, there is a substantial degree ofrecycling, requiring a substantial amount of energy. Thus, there is anongoing need for improved xylene isomerization catalysts and processes.

SUMMARY OF THE INVENTION

The present invention is directed to a process for the isomerization ofa para-xylene depleted, meta-xylene rich stream under at least partiallyliquid phase conditions using ZSM-23 with an external surface area of atleast 75 m²/g (indicating a small crystallite size), and a SiO₂/Al₂O₃ratio between 15 and 75 that produces a higher than equilibrium amountof para-xylene, i.e., more than about 24 wt % of para-xylene, based onthe total amount of xylenes. The catalyst used in the inventive processconverts meta-xylene to para-xylene while forming only a small amount ofortho-xylene.

In one embodiment, a C₈ aromatic hydrocarbon mixture comprisingpara-xylene, ortho-xylene and meta-xylene is provided to an ortho-xylenesplitter to produce a first stream comprising para-xylene andmeta-xylene and a second stream comprising ortho-xylene. The firststream comprising para-xylene and meta-xylene passes to a para-xylenerecovery unit to recover a para-xylene product stream and produce apara-xylene-depleted stream comprising meta-xylene, which is contactedwith a catalyst under at least partially liquid phase conditionseffective to produce a first isomerized stream having a para-xylenecontent of more than about 24 wt %, based on the total amount of xylenesin the first isomerized stream. At least a portion of the firstisomerized stream is then recycled back to the para-xylene recoveryunit. Optionally, the second stream comprising ortho-xylene is contactedwith a catalyst comprising ZSM-5 having an alpha value of at least 300under at least partially liquid phase conditions effective to produce asecond isomerized stream, at least a portion of which is recycled backto the ortho-xylene splitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one embodiment of the inventiveprocess.

FIG. 2 is a schematic representation of another embodiment of theinventive process.

FIG. 3 is a schematic representation of a third embodiment of theinventive process.

DETAILED DESCRIPTION

The present invention is directed to a process for the isomerization ofa para-xylene depleted, meta-xylene rich stream using ZSM-23 with anexternal surface area of at least 75 m²/g (indicating a smallcrystallite size), and higher aluminum content that produces a higherthan equilibrium amount of para-xylene, i.e., more than about 24 wt % ofpara-xylene, based on the total amount of xylenes. The smaller crystalsize and higher aluminum content allows the isomerization reaction to beconducted at lower temperatures than isomerization processes usinglarger crystal ZSM-23 catalysts, which in turn produces a better productyield.

With reference to FIG. 1, in a first embodiment of the inventiveprocess, a C₈ aromatic hydrocarbon mixture 100 comprising para-xylene,ortho-xylene and meta-xylene is provided to an ortho-xylene splitter110, which may be a fractional distillation column, selective sorptionunit, or any other technology known in the art. The C₈ aromatichydrocarbon mixture 100 may be derived from any C₈₊ aromatic hydrocarbonstream from which the ethylbenzene has been depleted or reduced by anymeans known in the art. The C₈ aromatic hydrocarbon mixture 100 may alsobe a C₈₊ aromatic hydrocarbon stream produced by a process that produceslow amounts of ethylbenzene, such as, but not limited to, a para-xyleneselective aromatic alkylation product stream, a non-selective(equilibrium para-xylene) aromatic alkylation product stream, anaromatic disproportionation stream, an aromatic transalkylation stream,a methanol/dimethyl ether to aromatic product stream, a syngas toaromatic product stream, a C₂-C₄ alkane/alkene to aromatic productstream, an import stream, and/or an offspec para-xylene stream from apara-xylene recovery unit.

The ortho-xylene splitter 110 separates a first stream 112 comprisingpara-xylene and meta-xylene from a second stream 114 comprisingortho-xylene. The first stream 112 comprising para-xylene andmeta-xylene is passed to a para-xylene recovery unit 120 to recover apara-xylene product 122 and leave a para-xylene-depleted stream 124comprising meta-xylene. Preferably, the para-xylene-depleted stream 124comprising meta-xylene consists essentially of meta-xylene. In oneembodiment, the para-xylene product stream comprises at least 50 wt %para-xylene, preferably at least 60 wt % para-xylene, more preferably atleast 70 wt % para-xylene, even preferably at least 80 wt % para-xylene,still even preferably at least 90 wt % para-xylene, and most preferablyat least 95 wt % para-xylene, based on the total weight of thepara-xylene product stream.

The para-xylene recovery unit 120 can include one or more of any of thepara-xylene recovery units known in the art, including, for example, acrystallization unit, an adsorption unit (such as a PAREX™ unit or anELUXYL™ unit), a reactive separation unit, a membrane separation unit,an extraction unit, a distillation unit, an extractive distillationunit, a fractionation unit, or any combination thereof. These types ofseparation unit(s) and their designs are described in “Perry's ChemicalEngineers' Handbook”, Eds. R H. Perry, D. W. Green and J. O. Maloney,McGraw-Hill Book Company, Sixth Edition, 1984, and thepreviously-mentioned “Handbook of Petroleum Refining Processes”.

The para-xylene-depleted stream 124 comprising meta-xylene is sent to ameta-xylene isomerization unit 130 where the meta-xylene stream 124 iscontacted with a xylene isomerization catalyst under at least partiallyliquid phase conditions effective to isomerize the meta-xylene stream124. Suitable conditions include a temperature of from about 400° F.(about 204° C.) to about 1,000° F. (about 538° C.), preferably fromabout 482° F. (250° C.) to about 572° F. (300° C.), more preferablyabout 482° F. (250° C.) to about 527° F. (275° C.); a pressure of fromabout 0 to 1,000 psig (6.9 MPa), preferably from about 350 psig (2.41MPa) to about 500 psig (3.45 MPa), more preferably about 350 psig (2.41MPa) to about 400 psig (2.75 MPa); and a weight hourly space velocity(WHSV) of from 0.5 to 100 hr⁻¹, preferably from 0.5 to 10 hr⁻1, morepreferably from 0.5 to 5 hr⁻1, with the pressure and temperature beingadjusted within the above ranges to ensure that at least part of themeta-xylene stream 124 is in the liquid phase. Generally, the conditionsare selected so that at least 50 wt % of the meta-xylene stream 124would be expected to be in the liquid phase.

The catalyst used in the meta-xylene isomerization unit 130 is a ZSM-23zeolite with a MIT structure type that has a SiO₂/Al₂O₃ ratio between15-75, preferably between 15-50, an external surface area of at least 75m²/g, preferably at least 90 m²/g, most preferably about 105 to 115m²/g, and an average crystal size of 5 microns or less, or 2 microns orless, or 1 micron or less, or 0.1 microns or less, such as thatdisclosed in U.S. Pat. Nos. 5,332,566; 4,599,475; and 4,531,012, whichare all incorporated herein by reference in their entireties.

External surface area may be calculated using the Brunauer-Emmett-Teller(BET) method. In the Brunauer-Emmett-Teller (BET) method, the overallsurface area (also referred to as total surface area) of a molecularsieve may be measured using the adsorption-desorption of nitrogen by asolid at 77 K as the function of relative partial pressure. The internalsurface area may be calculated using t-plot of theBrunauer-Emmett-Teller (BET) measurement. The external surface area iscalculated by subtracting the internal surface area from the overallsurface area measured by the Brunauer-Emmett-Teller (BET) measurement.

Particle size is measured by averaging the size of multiple particles asshown in SEM images obtained on a HITACHI S4800 Field Emission ScanningElectron Microscope (SEM). The particle size is measured by averagingthe size of multiple particles as shown in the SEM. The same method isused for crystal size. Transmission Electron Microscopy may also beused, but in event of conflict between SEM and TEM, SEM shall control.

In one embodiment, the ZSM-23 is self-bound.

In another embodiment, in addition to the zeolite, the catalyst employedin the meta-xylene isomerization unit 130 may include one or more binderor matrix materials resistant to the temperatures and other conditionsemployed in the process. Such materials include materials such as clays,silica, and/or metal oxides such as alumina. The latter may be eithernaturally occurring or in the form of gelatinous precipitates or gelsincluding mixtures of silica and metal oxides. Said materials maysuitably serve as diluents to control the amount of conversion in agiven process so that products can be obtained economically and orderlywithout employing other means for controlling the rate of reaction.These materials may be incorporated to improve the crush strength of thecatalyst under commercial operating to conditions. Said materials, i.e.,clays, oxides, etc., function as binders for the catalyst. It isdesirable to provide a catalyst having good crush strength because incommercial use it is desirable to prevent the catalyst from breakingdown into powder-like materials. These clay and/or oxide binders havebeen employed normally only for the purpose of improving the crushstrength of the catalyst and diffusion of reactants and products fromthe active sites in is the catalyst.

Naturally occurring clays which can be composited with the porouscrystalline material include the montmorillonite and kaolin family,which families include the subbentonites, and the kaolins commonly knownas Dixie, McNamee, Georgia, and Florida clays or others in which themain mineral constituent is halloysite, kaolinite, dickite, nacrite, oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment, or chemicalmodification.

In addition to the foregoing materials, the porous crystalline materialcan be composited with a porous matrix material such as silica, alumina,titania, zirconia, lanthanum oxide, yttrium oxide, zinc oxide,silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-beryllia, silica-titania as well as ternary compositions such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia,and silica-magnesia-zirconia. In addition other mixed metal oxides suchas hydrotalcite, perovskite, spinels, and inverse spinels may becomposited with the porous crystalline materials.

The relative proportions of porous crystalline material and optionalinorganic oxide matrix vary widely, with the content of the porouscrystalline material ranging from about 1 to about 90% by weight, andmore usually in the range of about 2 to about 80 wt % of the composite.In an embodiment in which the catalyst composition includes an inorganicoxide matrix material, the matrix material preferably comprises about 35wt % of alumina binder (making the balance of the catalyst compriseabout 65 wt % ZSM-23).

The meta-xylene isomerization unit 130 isomerizes the meta-xylene in thepara-xylene-depleted stream 124 and produces a first isomerized stream132 containing para-xylene at higher than its equilibrium amount, thatis at least about 24 wt % para-xylene, less than about 3 wt %ortho-xylene, with the balance being about 73 wt % meta-xylene, based onthe amount of meta-xylene sent to the meta-xylene isomerization unit130. Preferably, the meta-xylene isomerization unit 130 isomerizes themeta-xylene in the para-xylene-depleted stream 124 and produces at leastabout 27 wt % para-xylene, based on the total amount of xylenes. Anyortho-xylene remaining in the para-xylene-depleted stream 124 will passthrough the meta-xylene isomerization unit 130 unconverted; thus, theamounts above are based on a pure meta-xylene stream. At least a portionof the first isomerized stream 132 then is recycled to the para-xylenerecovery unit 120. At least a portion of the first isomerized stream 132may also be sent to the ortho-xylene splitter 110 as purge stream 134 toprevent the build-up of ortho-xylene in the meta-xylene isomerizationloop.

Because the meta-xylene isomerization unit 130 produces para-xylene inhigher than equilibrium amounts, as compared to the equilibrium amountsobtained by prior art catalysts, and the isomerization units are capableof operating in the liquid phase, the inventive process decreases therecycle necessary to produce the same amount of para-xylene, resultingin increased efficiency and energy savings.

The second stream 114 comprising ortho-xylene may be sold as product orsent to an ortho-xylene isomerization unit 140 where the second stream114 comprising ortho-xylene is contacted with a xylene isomerizationcatalyst under at least partially liquid phase conditions effective toisomerize the second stream 114 comprising ortho-xylene back towards anequilibrium concentration of the xylene isomers. Suitable conditionsinclude a temperature of from about 400° F. (about 204° C.) to about1,000° F. (about 538° C.), a pressure of from about 0 to 1,000 psig, aweight hourly space velocity (WHSV) of from 0.5 to 100 hr⁻¹, with thepressure and temperature being adjusted within the above ranges toensure that at least part of the second stream 114 comprisingortho-xylene is in the liquid phase. Generally, the conditions areselected so that at least 50 wt % of the second stream 114 comprisingortho-xylene would be expected to be in the liquid phase.

Any catalyst capable of isomerizing xylenes in the liquid phase can beused in the ortho-xylene isomerization unit 140, but in one embodimentthe catalyst comprises an intermediate pore size zeolite having aConstraint Index between 1 and 12. Constraint Index and its method ofdetermination are described in U.S. Pat. No. 4,016,218, which isincorporated herein by reference. Particular examples of suitableintermediate pore size zeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-22,ZSM-23, ZSM-35, ZSM-48, and MCM-22, with ZSM-5 and ZSM-11 beingparticularly preferred, specifically ZSM-5. It is preferred that theacidity of the zeolite, expressed as its alpha value, be greater than300, such as greater than 500, or greater than 1000. The alpha test isdescribed in U.S. Pat. No. 3,354,078; in the Journal of Catalysis, Vol.4, p. 527 (1965); Vol. 6, p. 278 (1966); and Vol. 61, p. 395 (1980),each incorporated herein by reference as to that description. Theexperimental conditions of the test used to determine the alpha valuescited herein include a constant temperature of 538° C. and a variableflow rate as described in detail in the Journal of Catalysis, Vol. 61,p. 395. The ortho-xylene isomerization unit 140 produces a secondisomerized stream 142 containing xylenes at their equilibrium ratio,that is about 55 wt % meta-xylene, about 22 wt % ortho-xylene, and about23 wt % para-xylene, based on the total amount of xylenes in the stream.The second isomerized stream 142 is recycled to the ortho-xylenesplitter 110.

FIG. 2 shows an embodiment of the inventive process involving theremoval of ethylbenzene. A C₈₊ aromatic hydrocarbon stream 200 isseparated into a C₈ aromatic hydrocarbon stream 212 and a C₉₊ aromatichydrocarbon stream 214 by a xylene splitter column 210. The C₈₊ aromatichydrocarbon stream 200 may be any hydrocarbon stream containing xylenesand ethylbenzene, such as, but not limited to, a reformate stream(product stream of a reformate splitting tower), a hydrocracking productstream, a xylene or ethylbenzene reaction product stream, an aromaticdisproportionation stream, an aromatic transalkylation stream, a Cyclar™process stream, and/or an import stream.

In one embodiment, the C₈ aromatic hydrocarbon stream 212 is passed toan ethylbenzene removal unit 220. The ethylbenzene removal unit 220 maybe a fractionation column or an adsorption unit equipped with anethylbenzene-selective adsorbent. The ethylbenzene-depleted mixedxylenes stream 222 is then passed to an ortho-xylene splitter 110 andfollows the embodiment described with reference to FIG. 1. Because it isexpected that ethylbenzene will be non-reactive with the ZSM-23, inanother embodiment, where ethylbenzene removal unit 220 is not present,there is an ethylbenzene purge stream 226 taken prior to the meta-xyleneisomerization unit 130 or an ethylbenzene purge stream 236 taken afterthe meta-xylene isomerization unit 130.

FIG. 3 shows another embodiment of the inventive process involving theremoval of ethylbenzene. As with the FIG. 2 embodiment, a C₈₊ aromatichydrocarbon stream 200 is separated into a C₈ aromatic hydrocarbonstream 212 and a C₉₊ aromatic hydrocarbon stream 214 by a xylenesplitter column 210. The C₈ aromatic hydrocarbon stream 212 is passed toan ethylbenzene conversion unit 320, where ethylbenzene is dealkylatedto benzene. Although ethylbenzene removal can be carried out in liquidphase, it is preferably achieved in gas phase. Hydrogen is fed to theethylbenzene conversion unit 320. Preferably, once-through low pressurehydrogen is used, thereby eliminating facilities for the recovery andrecycle of hydrogen. In the ethylbenzene removal unit, the preferredcatalyst is the first catalyst used in the dual bed catalyst systemdescribed in U.S. Pat. Nos. 5,516,956 or 7,663,010. However, othercatalytic processes that accomplish dealkylating ethylbenzene to benzeneknown to those skilled in the art could be utilized, such as the firstcatalyst used in the dual bed catalyst system described in U.S. Pat. No.7,271,118. The ethylbenzene removal process is preferably operated atconditions maximizing ethylbenzene conversion per pass, preferably>80 wt% conversion per pass, and even more preferably>90 wt % conversion perpass. Operating conditions for the ethylbenzene conversion unit 320 willalso be chosen as to minimize undesirable transalkylation reactionsleading to xylene losses to C₇, C₉, or C₁₀ aromatics.

The ethylbenzene-depleted stream 322 is then passed through a highpressure separator (not shown) to remove hydrogen-rich light gas beforeit is sent to a deheptanizer column 330. The overhead stream 332 of thedeheptanizer column 330 mostly contains C₆ and C₇ aromatic hydrocarbonsand may be sent to further processing. The bottoms stream 334 comprisingmixed xylenes is passed to an ortho-xylene splitter 110 and follows theembodiment described with reference to FIG. 1.

The invention will now be more particularly described with reference tothe following non-limiting Examples.

EXAMPLE 1 Synthesis of ZSM-5

ZSM-5 was made as disclosed in U.S. Pat. Nos. 3,702,886; 3,790,471;3,755,145; and 3,843,741, the disclosures of which are all incorporatedin their entireties. The synthesized ZSM-5 had a MFI structure type witha SiO₂/Al₂O₃ ratio between 40 and 60 and an average crystal size of lessthan 0.05 microns. The elemental analysis of the synthesized ZSM-5 isshown below in Table 1, determined by method AM-I 1073 in which theamount of silica, alumina, sodium and potassium in a catalyst sample isfound by inductively coupled plasma atomic emission spectroscopy(ICP-AES).

TABLE 1 Elemental analysis of ZSM-5 Component Wt % Alumina (Al₂O₃) 2.80Sodium (Na) <0.005 Silica (SiO₂) 88.3 Potassium (K) <0.01 Water (H₂O)Balance

EXAMPLE 2 Synthesis of ZSM-23-A

ZSM-23-A was made as disclosed in U.S. Pat. Nos. 5,332,566; 4,599,475;and 4,531,012, the disclosures of which are all incorporated in theirentireties. The resulting product has a XRD pattern equivalent to ZSM-23with the majority of the crystal having a crystallite size below 0.1microns as determined by transmission electron microscopy (TEM), andexternal surface area of 110 m²/g, and a SiO₂/Al₂O₃ ratio of about 35.This crystal was then extruded with 65% zeolite and 35 wt % Versalalumina, exchanged with an ammonium salt and calcined to prepare theacid from of the zeolite. The calcined extrudate was then sized to 40-60mesh for catalytic testing.

EXAMPLE 3 Comparison of ZSM-5 and ZSM-23-A for Xylenes Isomerization

The ZSM-5 as synthesized in Example 1 and the ZSM-23-A as synthesized inExample 2 were tested in a meta-xylene isomerization process usingsimilar conditions. The conditions used and results obtained are shownbelow in Table 2. The WHSV was chosen to maintain constant meta-xyleneconversion in order to accurately compare selectivity to para-xylene.

TABLE 2 Comparison of ZSM-5 and ZSM-23-A in meta-xylene isomerizationZSM-5 ZSM-23-A Catalyst (Example 1) (Example 2) Temperature (° C.) 260260 Weight Hourly Space Velocity 34 5.1 (hr⁻¹) Feed composition 99.8 wt% 99.8 wt % meta-xylene meta-xylene Pressure (bar-a/MPa) 1.6/0.161.3/0.13 Meta-xylene conversion (%) 7 7 Ratio ofpara-xylene/ortho-xylene 4.0 11.8 formed Selectivity to para-xylene (%)80 92

As Table 2 shows, the ZSM-23-A with a small crystal size is moreselective to para-xylene than ortho-xylene, and thus produces apara-xylene in an amount higher than its equilibrium concentration.

EXAMPLE 4 Synthesis of ZSM-23-B

ZSM-23-B was made as disclosed in U.S. Pat. Nos. 5,332,566; 4,599,475;and 4,531,012, the disclosures of which are all incorporated in theirentireties. The resulting product has a XRD pattern equivalent to ZSM-23with the majority of the crystal having a crystallite size of about 1-2microns as determined by scanning electron microscopy (SEM), externalsurface area of about 60 m²/g, and a SiO₂/Al₂O₃ ratio of about 73. Thecrystal was extruded with 0% binder.

EXAMPLE 5 Synthesis of ZSM-23-C

ZSM-23-C was synthesized as described above in Example 2, but theresulting crystals were not extruded with alumina binder. As in Example2, the resulting product has a XRD pattern equivalent to ZSM-23 with themajority of the crystal having a crystallite size below 0.1 microns asdetermined by transmission electron microscopy (TEM), external surfacearea of about 110 m²/g, and a SiO₂/Al₂O₃ ratio of about 35.

EXAMPLE 6 Comparison of ZSM-23-B and ZSM-23-C for Xylenes Isomerization

The ZSM-23-B as made in Example 4 and the ZSM-23-C as made in Example 5were tested in a meta-xylene isomerization process using similarconditions. The conditions used and results obtained are shown below inTable 3. The WHSV was chosen to maintain constant meta-xylene conversionin order to accurately compare selectivity to para-xylene.

TABLE 3 Comparison of ZSM-23-B and ZSM-23-C in meta-xylene isomerizationZSM-23-B ZSM-23-C ZSM-23-C Catalyst (Example 4) (Example 5) (Example 5)Temperature (° C.) 260 260 260 Weight Hourly Space 2.55 15.5 5.2Velocity (hr⁻¹) Feed composition 99.8 wt % 99.8 wt % 99.8 wt %meta-xylene meta-xylene meta-xylene Pressure (bar-a/MPa) 31/3.1 31/3.131/3.1 Meta-xylene conversion (%) 19 25 33 Amount of meta-xylene in 80.474.5 66.7 product (wt %) Amount of para-xylene n 17.6 23.3 28.7 product(wt %) Amount of ortho-xylene in 1.7 1.9 4.3 product (wt %) Selectivityto para-xylene (%) 91 92 86

As Table 3 shows, the smaller crystal size and higher SiO₂/Al₂O₃ ratioof the ZSM-23 affect the activity as more ZSM-23-B catalyst (lower WHSV)is necessary to achieve a certain conversion compared to ZSM-23-Ccatalyst. ZSM-23-C yields more para-xylene than ZSM-23-B at a givenyield of ortho-xylene, further lending support that smaller crystal sizeand higher SiO₂/Al₂O₃ lead to improved performance. The para-xyleneyield of 28.7 wt %, based on the total amount of xylenes, exceeds thatof prior art catalysts, i.e., ZSM-5. Additionally, the smaller crystalsize and lower SiO₂/Al₂O₃ ratio of ZSM-23-C (Example 5) provide thehighest para-xylene yield at lower WHSV with only slightly lowerpara-xylene selectivity. Without wishing to be bound by theory, giventhat isomerization is likely to be occurring in the pore mouth of theZSM-23, the smaller crystal gives more sites for the isomerization tooccur, allowing for the reaction to be run at higher space velocities.In addition, the increased aluminum content in the zeolite provides moreacid sites at the pore mouth, thereby creating more active sites in thezeolite. Coupling the small crystal size and increased aluminum contentis one possible explanation for the increase in the conversion whileminimizing any deleterious effects on the selectivity of the catalyst.

As seen by comparing Example 3 with Example 6, binding the ZSM-23 withalumina affects the conversion but not the selectivity. Example 3 usesZSM-23-A which is bound with alumina while Example 6 uses the sameZSM-23 without a binding. The unbound catalyst shows significantlybetter meta-xylene conversion, while maintaining a high para-xyleneselectivity.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

The invention claimed is:
 1. A process for producing para-xylene from a C8 aromatic hydrocarbon mixture, the process comprising: (a) providing a C8 aromatic hydrocarbon mixture comprising para-xylene, ortho-xylene and meta-xylene to an ortho-xylene splitter to produce a first stream comprising para-xylene and meta-xylene and a second stream comprising ortho-xylene; (b) passing the first stream comprising para-xylene and meta-xylene to a para-xylene recovery unit to recover a para-xylene product stream and produce a para-xylene-depleted stream consisting essentially of meta-xylene; (c) contacting the para-xylene-depleted stream comprising meta-xylene with a catalyst under at least partially liquid phase conditions effective to produce a first isomerized stream having a para-xylene content of more than 24 wt.%, based on the total amount of xylenes in the first isomerized stream; and first isomerized stream, wherein the catalyst comprises ZSM-23 with a SiO₂/A1 ₂O₃ ratio between 15 and 50 and an external surface area of 75-115 m²/g; and (d) recycling at least a portion of the first isomerized stream back to the para-xylene recovery unit.
 2. The process of claim 1, wherein the catalyst in step (c) is self-bound.
 3. The process of claim 1, wherein at least a portion of the first isomerized stream is recycled back to the ortho-xylene splitter.
 4. The process of claim 1, further comprising: (e) contacting the second stream comprising ortho-xylene with a catalyst comprising ZSM-5 having an alpha value of at least 300 under at least partially liquid phase conditions effective to produce a second isomerized stream; and (f) recycling at least a portion of the second isomerized stream back to the ortho-xylene splitter.
 5. The process of claim 4, wherein the liquid phase conditions of step (c) and (e) comprise a temperature from 400° F. (204° C.) to 1,000° F. (538° C.), a pressure of from 0 to 1,000 psig, a weight hourly space velocity (WHSV) of from 0.5 to 100 hr⁻¹.
 6. The process of claim 4, wherein the liquid phase conditions of step (c) comprise a temperature from 482° F. (250° C.) to 572° F. (300° C.), a pressure from 350 psig (2.41 MPa) to 500 psig (3.45 MPa), and a weight hourly space velocity (WHSV) from 0.5 to 10 hr⁻¹.
 7. The process of claim 1, wherein the C₈ aromatic hydrocarbon mixture further comprises ethylbenzene and the process further comprises a step of reducing the ethylbenzene.
 8. The process of claim 7, wherein the ethylbenzene is removed prior to the ortho-xylene splitter by an ethylbenzene removal unit or an ethylbenzene conversion unit.
 9. The process of claim 8, wherein the ethylbenzene removal unit comprises a fractionation column or an adsorption unit equipped with an ethylbenzene-selective adsorbent.
 10. The process of claim 8, wherein the ethylbenzene conversion unit comprises a catalytic system effective to dealkylate ethylbenzene to benzene and produce an ethylbenzene-depleted stream.
 11. The process of claim 7, wherein the ethylbenzene is removed by a purge stream from the para-xylene-depleted stream comprising meta-xylene prior to step (c) or from the second isomerized stream after step (c). 